The definitions that are already there seem pretty much right except that I've never heard anyone actually say "nose goes", what I've always seen and done is that you touch your nose and say "Not it".

Guy 1: "Someone go get a tape so I can record Tony Romo's bitch ass fumbling the snap on that field goal!"
Everyone but some other guy: "Not it!"(while touching nose, nose goes)
Other guy goes and gets it.